Conventional printing systems typically include several host computers and several printers, all connected to one another via a network, such as a local area network (LAN). Many modern printers, including laser printers and ink jet printers, contain a network adapter which allows such printers to interface into the network, and more recently, such printers have begun communicating with host computers via networks using a Network Printer Alliance Protocol (NPAP), which defines a bi-directional means for communication between host computers and printers. Communications using NPAP compliant data and commands are transported under various network Protocols, such as TCP/IP.
Details of the published NPAP are contained in the NPAP Specification Level 1, Revision N, dated Feb. 11, 1994. This NPAP specification is incorporated herein by reference. The employment of the NPAP means of communication is described in a number of examples for a single Protocol network adapter interface in a commonly assigned U. S. patent application, Ser. No. 08/350,860, U.S. Pat. No. 5,550,957; titled MULTIPLE VIRTUAL PRINTER NETWORK INTERFACE, filed Dec. 7, 1994, which is also incorporated herein by reference.
The network adapter interface described in the above patent application is an Internal Network Adapter (INA) which is plugged into an internal option port (an "IOP slot") of a preferred laser printer. A printer having a serial port or a parallel port can also be attached to a network via an External Network Adapter (ENA), which is the subject matter of the present invention. The advantage of using an Internal Network Adapter (INA) is that the INA can contain two channels, a "Normal Channel" and an "Alternate Channel," which can each carry data communications between the network and the printer, in which messages carried by these dual channels can be communicated simultaneously. If, for example, the printer is currently busy with a print job received through the Normal Channel from a host computer via the network, the same printer can simultaneously receive, for example, an NPAP query or an NPAP command through the Alternate Channel from the same host, or a different host computer, also through the same network. This gives the printer having an INA a great amount of flexibility in transmitting and receiving both data and commands through the network. An External Network Adapter cannot simply duplicate the INA's hardware and software, since the ENA communicates to a printer via a single channel connection (i.e., via a parallel port or a serial port). Even though the parallel or serial port is a hi-directional port, it can only transmit or receive a single data packet at any given moment.